1. Field of the Invention
The present invention relates to a filter apparatus including a slave gi-C filter formed by operational transconductance amplifiers (OTAs: gm) and capacitors (C) and a master circuit for automatically tuning the frequency characteristics such as a cut-off frequency or center frequency of the slave gm-C filter.
2. Description of the Related Art
Wide-dynamic-range gm-C filters formed by metal oxide semiconductor (MOS) OTAs and capacitors have been developed since 1984. For these filters, automatic tuning is required to maintain precise frequency characteristics of the gm-C filters in spite of manufacturing process variations, temperature drift and the like.
A first prior art filter apparatus formed on a large scale integrated circuit (LSI) chip is constructed by a slave gm-C filter and a master circuit formed by a phase-locked loop circuit for generating a control voltage for automatically tuning the frequency characteristics of the slave gm-C filter in accordance with a reference frequency signal see: P. Krummenacher et al., “A 4-MHz CMOS Continuous-Time Filter with On-Chip Automatic Tuning”, IEEE J. Solid-State Circuits, Vol. 23, No. 3, pp. 750–758, Jun. 1988). This will be explained later in detail.
In the above-described first prior art filter apparatus, however, since the operation mechanism of the voltage-controlled oscillator (VCO) of the master circuit is very complex, this filter apparatus including the VCO is also complex.
Additionally, in the above-described first prior art filter apparatus, parasitic capacitances in realized circuits cannot be ignored, so that it is impossible to maintain a precise relationship between the oscillation frequency of the VCO and the cut-off frequency or center frequency of the slave gm-C filter, particularly, in a low current type filter apparatus where the drive currents of the OTAs are small.
A second prior art filter apparatus formed on an LSI chip is constructed by a slave gm-C filter and a master circuit of a phase locked loop (PLL) type formed by a gm-C filter having the same structure as the slave gm-C filter (see: JP-9-320199-A). The master circuit receives a reference frequency signal and generates a control voltage for controlling the OTAs in the gm-C filter of the master circuit, so that the phase of the output signal of the gm-C filter of the master filter circuit is made equal to 90°. The control voltage is also used for controlling the slave gm-C filter, thus automatically tuning the frequency characteristics thereof. This also will be explained later in detail.
In the above-described second prior art filter apparatus, however, since the gm-C filter of the master filter circuit have the same or a similar structure to that of the gm-C slave filter, the operating frequency band of the gm-C filter of the master circuit is substantially the same as that of the slave gm-C filter. Therefore, the second prior art filter apparatus cannot be applied to a filter apparatus where the operating frequency band of a gm-C filter of a master circuit is different from that of a slave gm-C filter. Also, since the phase of the gm-C filter of the master circuit is required to be precisely 90° detected by a 90° phase detection circuit, the controllability is severe.